There have been known various methods of manufacturing an insulating box for a low-temperature show case having slidable transparent doors to cover its top opening, as shown in, for example, Japanese Patent Early Publication No. 60-165487. This Publication discloses a method of filling the spaces between the inner and outer boxes by injecting liquid state expansion insulating material into the space. In this method, if the height of a liner placed in the box during the injection of the insulating material is smaller than the height between the projecting wall of the insulating box that projects over the top opening and the opposite (bottom) wall, the injection is carried out in such a way that a gap is not formed between the liner and the wall of the insulating box.
A low-temperature show case according to the Publication is manufactured by installing a transparent window in the upper portion of one wall of the insulating box. The transparent window must be inserted into the groove formed in a plastic breaker after the expandable insulating material is filled in the space defined by the inner and outer boxes and said plastic breaker for connecting said boxes. However, the shape of the window receiving portion of the breaker is deformed as a result of outward bulging of the surfaces of the groove due to the pressure of the expanded insulating material, narrowing the mouth of the window receiving portion. Since the transparent window may not be then inserted into the narrowed groove, a process for correcting the groove of the breaker is needed before inserting the window, which is very tedious and sometimes impossible when the distortion of the groove is too great.